My object isn't showing up when I import it
If your object has been imported without errors, you should be able to see it in the outliner:



If it's there, select it in the outliner (left click on its name). 

You will be able to check its size in the "Number" panel at the right of the 3D viewport: if it's not showing, press N to open it. Look under "Transform">"Dimensions":



dimensions too small (smaller than 0.01) or too big (larger than 100) can be tricky to visualize. Scale your object up or down by using S and then moving the mouse until the dimensions are around 1.



You can also hit . to center your object into view.



If your object has dimensions of zero, there were probably some issues with the file format and/or the 3D information hasn't been imported properly. If that's the case, you should provide more information about the file format and the tools you are using to import it.